In this paper, martensitic transformation induced plasticity (TRIP) of 9Cr1Mo steel is analyzed by using a Gleeble 3500 test simulator. The values of transformation-induced plasticity under different applied loads are obtained by measuring the axial deformations of the specimen in a free dilatometric test and transformation plasticity test. The experimental results show that there is a good linear relationship between the final TRIP strain and the applied stress. On the basis of experimental results, some models describing TRIP due to the Greenwood-Johnson mechanism are considered. The TRIP coefficient, the evolution of TRIP strain during martensitic transformation as well as the dependence of TRIP strain on applied stress are all studied. The values of TRIP coefficient under different applied stresses are almost equal. As for the evolution of TRIP strain with the martensitic volume fraction, the description based on Leblond model is in good agreement with our experimental results.